1. Field of the Invention
The invention concerns a method of monitoring a flame such as a flame of a gas or oil burner and an apparatus for monitoring a flame.
2. Description of the Prior Art
Monitoring gas flames frequently entails the use of flame monitors which utilize the rectifier effect of the flame, that is to say which operate on the basis of what is known as the ionization principle. In that procedure an ac voltage is applied between two electrodes. The volume which is filled by the flame depends on the instantaneous output of the burner. The direct current which can be produced can be very low at a low level of burner output and if the geometry of the electrodes is not the optimum, while the alternating current can be substantially greater in dependence on the capacitance of the sensor line. The flame signal amplifier must therefore be capable of filtering off the low direct current component in the overall sensor circuit current, without the alternating current being able to simulate a flame signal as a result of the inevitable rectifier effects in the amplifier input. Therefore the magnitude of the direct current component gives a measurement in respect of the intensity of the flame, in which respect the absence of a flame corresponds to the intensity of zero, the detection of which must be established reliably and very close to real time in order to avoid unburnt gas or oil from flowing out into the burner chamber.
In principle filtering of the direct current component can be implemented by an evaluation circuit which is connected upstream of the flame signal amplifier, such as for example a low pass filter with a sufficiently low limit frequency. If however the filter capability of the low pass filter is lost, for example because of a failure of a filter capacitor, the alternating current could simulate the presence of a flame, even when the flame is not present. The flame monitoring or burner control system must recognise that malfunction. In the case of burners involving intermittent operation, that is normally not a problem because, after the fuel supply is switched off, which results in the flame being extinguished, the control system can detect a simulated flame signal as being a defect and can prevent the burner from being set in operation again. In the case of burners in continuous operation the malfunction must be detected by periodically checking the flame monitor without the burner being taken out of operation for that purpose. In the case of optical flame sensors, this is generally effected by interrupting the beam path between the flame and the sensor by means of a shutter member, that is to say flame failure is temporarily simulated during operation of the assembly, and the output of the flame signal amplifier must suitably react thereto.
In principle the method of signal interruption at the flame sensor can also be used in regard to ionization flame monitoring. The ionization circuit could be interrupted by means of a suitable switch element. However that element would have to be disposed close to the sensor electrode so that only the flame signal current is interrupted and not for example also the alternating current which flows by way of line capacitances and whose flame-simulating effect in the event of a component fault is in fact precisely to be detected by the test. It would also be possible to envisage short-circuiting of the flame signal lines whereby the flame signal current also becomes zero, and the alternating current is even increased. For both cases, it would be necessary to use a switch element which is suitable for the high sensor ac voltage and which itself cannot assume a fault mechanism which results in undetected flame simulation.
In the present state of knowledge only an electromechanical relay can be considered for that purpose. That structure however is expensive in terms of material and equipment and requires a relatively high level of control power. The possibility of interrupting the sensor current with a relay contact is mentioned in German laid-open application (DE-OS) No 29 32 129 on page 6. DE 30 26 787 describes a construction in which there is a single filter capacitor at the input of the flame signal amplifier, which on the one hand serves as an energy storage means for the ionization current and whose discharge current on the other hand is required for dynamic operation of a semiconductor circuit. Failure of that filter capacitor has the consequence that the semiconductor circuitxe2x80x94even in the case of an alternating current caused by sensor line capacitancesxe2x80x94goes into a constant state so that a flame is no longer signalled. The disadvantage of that arrangement is that a given minimum level of energy and thus a given minimum current must be supplied by the flame for dynamic operation of that semiconductor circuit. Therefore certain limits are set on the response sensitivity of that circuitry principle and it no longer satisfies all present-day requirements.
EP 159 748 discloses a circuit which leads to the assumption of a high level of response sensitivity insofar as the capacitive load current caused by line capacitances, at the sensor terminals, remains low in comparison with the flame signal current. In that respect this circuit does not satisfy the demands for a high level of response sensitivity and at the same time a high level of resistance in relation to line capacitance. A further requirement which is frequently specified is the display of flame intensity as a setting aid when bringing a burner into operation and for detecting changes of the flame in operation, in good time. The circuit disclosed in EP 159 748 does not afford that option.
The arrangement in accordance with the teaching of DE 30 26 787 supplies a pulse series in dependence on the magnitude of the flame current so that in that case it would be possible to derive a signal to indicate the flame intensity, but the dynamic range between response sensitivity and saturation limit is relatively small so that the circuit principle is only suitable for establishing xe2x80x9cflame presentxe2x80x9d.
EP 0 617 234 also discloses an ionization flame monitor with a circuit arrangement having a capacitor which is transferred by the ionization current from a condition of being charged up by the operating voltage into a discharged condition, wherein the signal xe2x80x9cflame presentxe2x80x9d is output when the value falls below a given threshold. The function of the capacitor can be checked by means of a test signal. The disadvantage here is that the function of the capacitor has to be periodically tested, the system does not provide for continuous monitoring of the capacitor.
Therefore the object of the present invention is to provide a method and an apparatus for monitoring a flame which serves as a flame monitoring method and circuit respectively, the response sensitivity of which is substantially improved in comparison with the state of the art without detracting from compatibility for line capacitance, whose switch-off capability can be periodically checked during burner operation and also supplies an output signal representing a measurement in respect of flame intensity. The invention further seeks to provide that the method ensures continuous checking of the monitoring action.
According to a first aspect of the present invention, there is provided a method of monitoring a flame, wherein:
in dependence on the presence or intensity of the flame there is produced from a first electrical signal a second electrical signal of different magnitude,
the second electrical signal is applied to an evaluation circuit and converted into a first output signal, and
the evaluation circuit is acted upon by a monitoring signal which upon failure of the evaluation circuit leads to a second output signal.
According to a second aspect of the present invention, there is provided apparatus for monitoring a flame, comprising:
an evaluation circuit which in dependence on the presence or the intensity of the flame generates from a first electrical signal a second electrical signal of different magnitude, and
circuit means which converts the second electrical signal into a first output signal,
wherein the evaluation circuit can be acted upon with a monitoring signal which in the event of failure of the evaluation circuit leads to a second output signal.
Advantageous configurations are set forth in the dependent claims.
The method according to the invention of monitoring a flame makes use of the known principle that in dependence on the presence or the intensity of the flame there is produced from a first electrical signal (for example an ac voltage signal) a second electrical signal of different magnitude (for example a dc signal) (IF) . A preferred embodiment uses ionization electrodes or ultraviolet sensors with series-connected diode which in dependence on flame intensity supply a corresponding dc signal. No dc signal is produced when the flame is extinguished. The second electrical signal (IF) is detected by an evaluation circuit to which the ionization electrodes or the ultraviolet sensors are connected, and converted into a first output signal (A), wherein conversion is effected by various further circuit elements in such a way that differently dynamic output signals are obtained, depending on the respective flame intensity involved. Therefore with changing flame intensities the output signal (A) is an output signal which changes in terms of its dynamics.
The evaluation circuit is also acted upon by an electrical monitoring circuit (ac voltage signal) which can be derived for example from the ac voltage signal made available to the ionization electrodes, which upon failure of the evaluation circuit results in a second output signal (AA). That second output signal, as in the case of flame failure, is advantageously a static signal, so that the monitoring apparatus immediately notes the failure of the evaluation circuit and can cause the fuel supply to be shut off.
The second electrical signal (IF) is converted into a control signal (S) and passed on to a trigger stage. This trigger stage can be for example an operational amplifier which compares the control signal to a given threshold and then resets the evaluation circuit again by way of a reset signal (R) so that it can again control the trigger stage. In that way the output of the trigger stage is switched over between two output signals (A1, A2) in dependence on the control signal (S). The trigger stage switches to and fro at different rates in dependence on the flame intensity.
The control signal (S) can also be passed by way of a further evaluation circuit in order to improve the sensitivity of the circuit in relation to the second electrical signal, that is to say for example in relation to the direct current component in the sensor current. In order to check that second evaluation circuit, that is to say in order to detect a failure, its circuitry is connected to the control input of an integrator which is in the form of a charge pump and whose output signal reflects the magnitude of the second electrical signal, such as for example of the sensor current.
A monitoring circuit serves to detect a failure of the first evaluation circuit, the monitoring circuit being supplied by way of the evaluation circuit with a monitoring signal, that is to say for example an ac voltage signal, so that in the event of failure of the evaluation circuit the monitoring circuit is taken out of operation and that results in a static output signal (AA). The output signal of the integrator becomes zero in the event of failure of the sensor current.